And I stared through that obscurity,
I saw what seemed a cluster of great towers,
Whereat I cried, “Master, what is this city?”
—Dante, Canto XXXI, The Inferno

The first person to photograph the underground of Paris was a gallant and theatrical man with a blaze of red hair, known as Nadar. Once described by Charles Baudelaire as “the most amazing example of vitality,” Nadar was among the most visible and electric personalities in mid-nineteenth-century Paris. He was a showman, a dandy, a ringleader of the bohemian art world, but he was known especially as the city’s preeminent photographer. Working out of a palatial studio in the center of the city, Nadar was a pioneer of the medium, as well as a great innovator. In 1861, Nadar invented a battery-operated light, one of the first artificial lights in the history of photography. To show off the power of his “magic lantern,” as he called it, he set out to take photographs in the darkest and most obscure spaces he could find: the sewers and catacombs beneath the city. Over the course of several months, he took hundreds of photographs in subterranean darkness, each requiring an exposure of eighteen minutes. The images were a revelation. Parisians had long known about the cat’s cradle of tunnels, crypts, and aqueducts beneath their streets, but they had always been abstract spaces, whispered about, but seldom seen. For the first time, Nadar brought the underworld into full view, opening Paris’s relationship to its subterranean landscape: a connection that, over time, grew stranger, more obsessive, and more intimate than that of perhaps any city in the world.

A century and a half after Nadar, I arrived in Paris, along with Steve Duncan and a small crew of urban explorers, with an aim to investigate the city’s relationship to its underground in a way no one had before. We planned a traverse — a walk from one edge of the city to the other, traveling exclusively by subterranean infrastructure. It was a trip Steve had dreamed up back in New York: we’d spent months planning, studying old maps of the city, consulting Parisian explorers, and tracing potential routes. The expedition, in theory, was tidy. We would descend into the catacombs just outside the southern frontier of the city, near Porte d’Orléans; if all went according to plan, we’d emerge from the sewers near Place de Clichy, beyond the northern border. As the crow flies, the route was about six miles, a stroll you could make between breakfast and lunch. But the subterranean route — as the worm inches, let’s say — would be winding and messy and roundabout, with lots of zigzagging and backtracking. We had prepared for a two- or three-day trek, with nights camping underground.

Every step of the trip, of course, would be illegal.

On a mild June evening, six of us sat on the southern boundary of the city, in a derelict train tunnel that was part of the petite ceinture, or the “little belt,” a long-abandoned train track that encircles Paris. We’d spent the day collecting last-minute supplies: now it was past nine, and the dots of light at either end of the tunnel were darkening. Everyone was quiet, our headlamp beams dancing anxiously over the floor. We took turns peering down into a dark, graffiti-ringed hole jack-hammered out of the concrete wall, which would be our entrance into the catacombs.

“Best to keep passports in a zipper pocket,” said Steve, thumbing the braces on his waders. “Just in case.” Every step of the trip, of course, would be illegal: if we got caught, having our IDs at the ready might be just enough to keep us out of Paris’s central lockup.

Moe Gates crouched over a map that would help us navigate the sprawling, mazelike tunnels of the catacombs. Short, bearded, and clad in a red Hawaiian shirt, Moe was Steve’s longtime exploring partner. He had run the sewers in Moscow, crouched on the gargoyles at the top of the Chrysler Building in Manhattan, and once had sex on the top of the Williamsburg Bridge in Brooklyn. He wanted to retire from exploring tunnels, to settle down and “have babies with a nice Jewish girl,” but he hadn’t been able to kick the habit.

Liz Rush — Steve’s girlfriend, a sharp-eyed woman with chestnut hair cropped above her shoulders — was checking batteries on a confined-space gas detector, which would alert us to any poisonous air that we might encounter in the unventilated tunnels. Liz had explored under New York with Steve, but this was her first trip beneath Paris. Sorting through gear next to Liz were two other first-timers: Jazz Meyer, a young Australian woman with red dreadlocks, who had explored storm drains under Melbourne and Brisbane; and Chris Moffett, a philosophy graduate student in New York, who would be making his first foray underground.

“Fifty percent chance of precipitation,” said Steve, checking his phone one last time before turning it off. The greatest threat to our trip was rain: once we reached the sewer collectors, even a small cloudburst on the surface could create a flood underground. It had been a wet June in Paris and, since our arrival in the city, we’d been obsessively monitoring the weather. Steve had enlisted a fellow explorer in the city, Ian, to text us weather updates. As a group, we’d made a vow: at the first sign of raindrops, we’d bail, expedition over.

As we huddled around the entrance, Moe, who would play the role of recordkeeper, checked his watch and made a note on a pad: “Nine forty-six p.m., underground.” Steve went first, snaking his hips through the entrance, legs scissor-kicking behind him; the rest of us followed, one after the next. I was last: I looked up and down the empty rail tunnel, took a deep breath, then squeezed down into the dark.

The tunnel we dropped into was narrow and low, with walls of raw, clammy stone. I slung my pack around to my chest and crawled on all fours, my back scraping against the rocky ceiling, while cold water sloshed around my hands and knees, soaking me to the skin. The stone gave off an earthy, almost pastoral aroma, like rain-soaked chalk. Our headlamp beams flitted in an arrhythmic strobe. So abrupt was the feeling of departure from the surface, we might as well have been at the bottom of the ocean. The honking of cars on the street, the rattle of the tram on avenue du Général Leclerc, the murmur of Parisians smoking under the awnings of brasseries — all were stamped out.

We headed north, with Steve in the lead. Down a wider gallery, we rose into a squelching duck-walk, then down an arched passage, with earthen ground underfoot, until all of us were up and marching, the first leg of our traverse under way.

Parisians say their city, with its galaxy of perforations, is like a great hunk of Swiss cheese, and nowhere is so holey as the catacombs. They are a vast, stony labyrinth, two hundred miles of tunnels, mainly on the Left Bank of the Seine. Some of the tunnels are flooded, half-collapsed, riddled with sinkholes; others are adorned with neatly mortared brick, elegant archways, and ornate spiral staircases. The “catas,” as they are known to the familiar, are technically not catacombs, a word usually traced back to an amalgam of the Greek katá- (down) and Latin tumbae (tombs); they are quarries. All of the stately buildings along the Seine — Notre-Dame, the Louvre, the Palais Royal — were erected of limestone blocks chopped from beneath the city. The oldest tunnels had been carved to construct the Roman city of Lutetia, traces of which could still be found in the city’s Latin Quarter. Over the centuries, as the city grew, stonecutters brought more limestone to the surface, and the underground warren expanded, fanning out beneath the city like the roots of a great tree.

In the years before Nadar first brought his camera beneath Paris, the quarries were silent. The only regular visitors were a handful of city laborers — the workers in the ossuary, who raked bones back and forth over the catacomb floors; the employees of the Inspection générale des carrières, who walked the stone passages by lantern light, bracing the tunnels to prevent them from collapsing under the city’s weight — and the occasional mushroom farmer, who took advantage of the dry, dark environment to grow his crop. For the rest of the city, the quarries were a blind spot: a distant place, a landscape more imaginary than real.

From the moment we went underground, so many years after Nadar, we could feel the quarries teeming. The walls tumbled with bright graffiti, and the mud floors were tracked up and down with footprints. When we came to shallow pools, the water swirled with mud, a sign of recent passers-through. These were traces of the cataphiles, a loose affiliation of Parisians who spent days and nights roaming the catacombs. A subtribe in the urban explorer kingdom, cataphiles were mostly college kids in their teens and twenties; some, however, were in their fifties and sixties, had been exploring the network for decades, had even raised cataphile children and grandchildren. The city employed a squadron of catacomb policemen — known as cataflics, literally “catacops” — who patrolled the tunnels and doled out sixty-five-euro tickets to trespassers. But they offered little deterrence to the cataphiles, who treated the tunnels like a giant secret clubhouse.

We’d been underground for about two hours when Steve led us through a tunnel so tight and low we dropped to our bellies and squirmed on our elbows through the dirt. As we popped out on the other side, we saw three headlamp beams bobbing in the dark. It was three young Parisians — cataphiles — led by a tall, rangy, dark-haired man in his mid-twenties named Benoit.

“Welcome,” he said, with a flourish, “to La Plage.”

We’d emerged in one of the main cataphile haunts, a cavernous chamber with sand-packed floors and high ceilings supported by thick limestone columns. Every surface — every inch of the wall, of the pillars, and much of the rocky ceiling — was covered in paintings. In the darkness, the paintings were subdued and shadowy, but under the beam of a flashlight, they blazed. The centerpiece was a replica of Hokusai’s Great Wave off Kanagawa, with the curling wave of frothy blues and whites. Spread throughout the room were stone-cut tables, rough-hewn benches and chairs. At the center of the chamber was a giant sculpture of a man with arms raised to the ceiling, like a subterranean Atlas, holding up the city.

“This is like — ” Benoit paused, apparently searching for a recognizable analogy “ — the Times Square of the catacombs.”

On weekend nights, he explained, La Plage and certain other voluminous chambers in the catacombs filled with revelers. Sometimes they’d siphon electricity from a lamppost on the surface and set up a band or a DJ. Or a cataphile would strap a boom box to their chest and go weaving through the tunnels, roaming from chamber to chamber, as the party followed, dancing in the dark, passing bottles of whiskey up and down, like a snaking subterranean conga line. Other gatherings were more urbane: you might turn in to a dark chamber to find a candlelit holiday party, with cataphiles drinking champagne and eating galette des rois.

So abrupt was the feeling of departure from the surface, we might as well have been at the bottom of the ocean.

Cataphiles had long flocked underground to make art, to paint and sculpt and build installations in hidden caverns. Not far from La Plage was the Salon du Chateau, where a cataphile had carved from stone a beautiful replica of a Norman castle and installed gargoyle sculptures in the wall. And the Salon des Miroirs, where the walls of a chamber were covered in a disco-ball mosaic of reflective shards. And La Librairie, a small nook with hand-carved shelves, where people could leave books for others to borrow. (The books, unfortunately, often grew moldy in the dank air.)

To wander through the catacombs is to feel yourself inside of a mystery novel, full of false walls and trapdoors and secret chutes, each leading to another hidden chamber, containing another surprise. Down one passageway, you might find a chamber containing a sprawling Boschian mural that cataphiles had been gradually embellishing for decades; down another, you might see a life-size sculpture of a man half inside a stone wall, as though stepping in from the beyond; down yet another, you might encounter a place that upends your very sense of reality. In 2004, a squadron of cataflics on patrol in the quarries broke through a false wall, entered a large, cavernous space, and blinked in disbelief. It was a movie theater. A group of cataphiles had installed stone-carved seating for twenty people, a large screen, and a projector, along with at least three phone lines. Adjacent to the screening room were a bar, lounge, workshop, and small dining room. Three days later, when the police returned to investigate, they found the equipment dismantled, the space bare, except for a note: “Do not try to find us.”

Whether or not they knew it, the cataphiles were essential to our traverse. Our map, which had been designed by the tribe’s elders, was a product of generations of cataphile knowledge: it marked which passages were low and necessitated a crawl, which were flooded, which had hidden pitfalls that would require careful stepping. (Wary of making the network too navigable, the elders left all entrances on the map unmarked.) Meanwhile, cataphiles over the years had brought power drills and jackhammers underground to gouge out small passages from the walls: chatières — “cat ways” — which would be vital gateways in our trek.

Benoit, who wore only a small bag to hold a bottle of water and an extra light, eyed our bulging packs. “How long do you plan to stay?” he asked.

Benoit stared at Steve for a moment, then laughed, evidently assuming it was a joke, before turning and heading off into the dark.

*

We twisted, squirmed, and crawled, contorting our bodies, as though performing an extended subterranean gymnastics routine. We squeezed through long constrictive passages, emerging in a tangle of limbs, like a newborn foal.

We climbed down into chambers the size of ballrooms, where our voices reverberated off the ceilings. The walls were slick with condensation and gave off steam: it was like moving through intricate foldings of brain tissue. We peered up manhole shafts seventy feet high, where it was too dark to see the top. Brown roots crept down from the ceiling like small, craggy chandeliers. The main tunnels were marked with Paris’s signature blue ceramic signs, the names corresponding to the streets above. In a realm of palimpsests, the graffiti from spray cans of cataphiles obscured smoke streaks from torches of seventeenth-century quarry diggers, which obscured fossils of ancient sea creatures embedded in the limestone. Every few moments, we passed tunnels branching off on either side, a reminder of just how tangled our path would become.

Chris, Liz, and Jazz, the first-timers, walked as though in a dream. “I can’t believe this place is real,” whispered Jazz.

At one point, I shined my light upward to find a giant black crack in the ceiling. In the eighteenth century, there had been collapses: buildings and horse-drawn carriages and people walking in the street swallowed by the earth as the stonecutters below were buried in rubble. But today the tunnels were secure and we did not fear entombment — the catacombs were the least perilous leg of our journey.

*

Long before Nadar began roaming the underside of Paris, he strove to photograph the world from unexplored perspectives: first, from the air. Together with his close friend Jules Verne, Nadar founded the Société d’encouragement de la locomotion aérienne au moyen d’appareils plus lourds que l’air (Society for the Encouragement of Aerial Locomotion by Means of Heavier-than-Air Machines), and launched spectacular hot-air balloon missions all over Europe. In 1858, he boarded a balloon and rose above Paris, where, from a height of 258 feet, he took the world’s first aerial photograph, a gently blurred silver-gray image of the city. “We have had bird’s-eye views seen by the mind’s eye imperfectly,” he wrote of the aerial mission. “Now we will have nothing less than the tracings of nature herself, reflected on the plate.”

Now, for his next trick, Nadar would photograph the city from below. It began with the arc lamp, which he had assembled in his studio. It was a powerful, if unwieldy, contraption: upon activating a rig of fifty Bunsen batteries, an electric current sparked two carbon rods, which sent out a flare of white light. The lamp made it possible to create images without natural sunlight, a novel concept in the nascent medium of photography. In the evenings, he’d set off the lamp on the sidewalk in front of his studio, drawing crowds with the blaze. Nadar declared that he would use his lamp to capture vistas with his camera that eluded all other photographers. “The world underground,” he wrote, “offered an infinite field of activity no less interesting than that of the top surface. We were going into it, to reveal the mysteries of its deepest, most secret caverns.” It was in the ossuaries — called Les Catacombes, in imitation of the famous catacombs in Rome — that Nadar took his first subterranean photographs.

Seven hours or so into our journey, Steve guided us down a long passageway, and into a chamber of cobbled walls. Everyone unshouldered their packs and took a rest on the floor. Morale was high, despite wet feet and a considerable accumulation of mud. It was several moments before we identified the dry, copper-colored objects strewn over the ground at our feet.

Jazz picked one up and studied it, her dreadlocks shifting on top of her head. “It’s a rib,” she said, letting it fall from her hand.

Sure enough, we looked down to find that we were stepping over bones — a tibia, a femur, the crown of a skull, each desiccated and smooth, the color of parchment. We ducked around a corner and found ourselves at the foot of a giant tower: thousands of bones in a jumbled cascade, spilling down a chute from the surface. We were standing in an ossuary beneath the Cimetière du Montparnasse.

At the end of the eighteenth century, Paris was overflowing with corpses. The walls of the Cimetière des Saints-Innocents, the largest burial ground in the city, buckled, and corpses tumbled into the basements of neighboring homes. In order to prevent the spread of disease, the city decided to relocate its dead to the underground quarries, which had been expanding beneath their feet for decades. The chosen site was a three-acre stretch of empty passageways in the south, beneath a street called Tombe-Issoire. After a trio of priests had descended underground to officially consecrate the tunnels, the bones began their trip across the city: they were transported in wooden carts draped in black veils, then dumped into pits in the streets. In all, the remains of six million dead made the trip into the quarries. Laborers were sent underground, and given the interminable task of sorting the bones and arranging them in intricate friezes.

A squadron of cataflics on patrol in the quarries broke through a false wall, entered a large … movie theater … Three days later, when the police returned to investigate, they found the equipment dismantled, the space bare, except for a note: ‘Do not try to find us.’

In December of 1861, Nadar — along with a team of assistants and two mining carts creaking with camera equipment — descended into the bone-lined corridors. The galleries had been briefly open to visitors in 1810, but were closed soon after due to vandalism; when Nadar arrived, the tunnels had been shuttered for decades. In the “molehills,” as he called them, Nadar encountered a crew of subterranean workers, toiling among the bones.

At the time, the process of taking a photograph even in the controlled environment of a studio was complicated; underground, in pitch-dark galleries, it was nearly impossible. The delays were maddening: the collodion formula would spill in the dark; the arc lamp would get stuck in tight corridors; the batteries would emit noxious fumes, which, in confined spaces, made everyone ill. As each shot required an eighteen-minute exposure, a full day of work brought few photographs; an assistant was heard to grumble, “We grow old down here.” But Nadar worked furiously. As his model he cast a wooden mannequin, which he outfitted with a beard, hat, boots, municipal coveralls, and a pitchfork used to rake bones.

Nadar produced seventy-three photographs of the ossuaries, a uniquely quiet and surreal collection. One showed a freshly pitched pile of bones; others focused on the intricate bone friezes, or on mannequin-workers pushing bone-filled wagons through corridors. From the moment they went on display at the Société française de photographie, the images were a sensation. Critics wrote of Nadar as a mythical figure, traversing the cosmos of the city. An article in the Journal des débats called the photographer “Beelzebub,” the lord of the underworld; another referred to him as a necromancer who had “electrified the mortal remains of past generations.” A whole secret dimension of the city had been unveiled: “He and his assistants,” wrote one journalist, “now in the bowels of the harmless earth, will enable people to become familiarized with scenes which but few have witnessed.” Nadar became the toast of salons and cafés, as the city talked incessantly of the subterranean images.

But it wasn’t only talk. The photographs awakened something in Parisians: having glimpsed the city’s underside, they wanted to touch and smell the tunnels for themselves, to hear their own footsteps in the dark. Around the same time the photographs were first displayed, the catacombs were reopened, rapidly becoming one of the city’s premier attractions. A few times a month, then more frequently, men in top hats and ladies in long dresses would wander through the ossuary in huddled groups, peering into hollow eye sockets of browned skulls, watching walls of stacked tibiae ripple in the candlelight. They shivered at the unearthly acoustics and the sensation of being under the damp earth; at the end of the tour, visitors would surreptitiously pluck skulls from the walls, stealing souvenirs from the underworld. So popular became the catacombs that in 1862, when Gustave Flaubert visited with the novelists Jules and Edmond de Goncourt, they grew cranky about the crowds. “One must put up with all those Parisian jokers,” wrote the famously acerbic Goncourt brothers, “who go underground on veritable pleasure parties and amuse themselves by hurling insults into the mouth of Nothingness.”

There was, too, a rush of unsanctioned visitors — proto-cataphiles — who pressed into the stretches beyond the route of the tour. Lovers arranged underground trysts; teenagers struck out on exploratory missions. Just as their cataphile descendants would do many years later, a group of Parisians held a clandestine concert in the catacombs. One hundred guests gathered on rue d’Enfer, having parked their carriages down the street so as not to arouse suspicion, then slipped down through the entrance. Sixty feet beneath the city, amidst candles burning atop human skulls, the guests sat before an orchestra of forty-five musicians. The evening’s bill included “Funeral March” by Chopin and Saint-Saëns’s Danse Macabre.

*

We camped an hour north, in a boxy chamber excavated at some point in the nineteenth century. From iron rings in the walls, everyone strung up hammocks, while Liz and I made spaghetti with tuna fish. We ate in silence, happy and exhausted. It was like camping on the moon: no sounds down here, nothing alive, just miles of darkness.

As we prepared for bed, Chris asked what time it was. Moe pointed out that we were in a place that had been perfectly dark and exactly 57 degrees Fahrenheit since it was created, a space untouched by all natural rhythms. “It’s never o’clock,” he said.

I awoke to find a woman standing in the doorway of our chamber. In one hand, she held an antique wrought-iron lantern with a hissing, naked flame that emitted honey-colored light. I watched her tiptoe into the center of the room, where she placed what appeared to be a small postcard on the floor.

“Bonjour,” I said, startling her.

Misty was in her forties and had been visiting the quarries since she was sixteen. Tonight she’d been wandering the tunnels alone — sans map, I noticed.

“Sometimes it’s nice to just go down for a walk,” she said in a lilting accent. Somehow her boots looked spotless, her gray blouse freshly dry-cleaned. As she walked from chamber to chamber in the quarries, Misty left little paintings wherever she went, like small messages to other cataphiles. The image she’d placed in our chamber showed two hands making a triangle.

*

It was 1 a.m. when we found our exit point from the catacombs: a chatière, barely wider than my shoulders. We were in a corner of the quarries seldom visited, where the ceiling was braced with centuries-old wooden bars installed long ago by the Inspection générale des carrières. We’d now been underground for twenty-seven hours. I had dried mud in the folds of my ears, the rims of my nostrils.

“I feel like I’m turning into a troglodyte,” said Liz, stretching out her legs in the tunnel.

“I keep picking stuff out of my hair that I don’t recognize,” said Jazz, studying a dreadlock. “I think I just found bone marrow.”

Moe removed his sock, took out a small vial of iodine, and started brushing the bright orange liquid into his toenail cuticles. Steve blinked at him.

“You think I’m not going to sterilize my hangnail before the sewers?”

To get to the sewers, we first had to negotiate a stretch of utility tunnels that would take us under the Seine. If the catacombs were the city’s cerebellum, the concrete tunnel we emerged into was a vein, a modest conduit linking more intricate organs. As we walked, it became clear just how close we were to the surface: filtering down from the street were the sounds of people chattering, high heels click-clacking, a dog barking. Through a vent in the wall, I caught an orange glow — the lights from an underground parking garage. I crouched and watched a woman with dark hair get into her car, back out, and drive away, and I felt like a ghost peering in on the city of the living.

We were unable to find a direct connection to the utility tunnel under the Seine, and so we had to go aboveground, but only for a moment. At the bottom of a manhole shaft with a ladder leading to the surface, we discussed the choreography of the exit in anxious whispers.

We emerged near Saint-Sulpice, in front of a store selling luxury baby clothing. With no police in sight, we set off zigzagging through empty alleys, moving in the direction of the Seine. At the end of a deserted street, Steve crouched and popped a hatch, and we all slipped back underground. As I lowered myself down, I caught the eye of a late-shift busboy holding salt and pepper shakers, a look of bewilderment on his face.

So popular became the catacombs that in 1862, when Gustave Flaubert visited with the novelists Jules and Edmond de Goncourt, they grew cranky about the crowds.

The tunnel under the Seine was damp, with dismal, submarine acoustics. Even here, we found evidence of interlopers: traces of graffiti, an empty liter bottle of Kronenbourg beer. As we crossed beneath the river, I imagined a cross-section view of the city, showing each stratum, layered one on top of the other. Above us, the towering silhouette of Notre-Dame, the bridges, the river. Deep below, the tunnels of the metro, which would soon be teeming with commuters. And there we were in the middle layer, six tiny cones of light cutting through darkness.

*

The dark and twisting sewers had, in the days before Nadar, been a source of irredeemable dread for Parisians. In Victor Hugo’s Les Misérables, which was written during the two decades preceding the release of Nadar’s photographs, the sewers embodied a kind of composite urban nightmare. The “intestine of the Leviathan,” Hugo wrote, is “tortuous, with cracks and torn-up cobblestones and ruts and strange bends, rising and descending for no obvious reason, fetid, wild, ferocious, sunk in darkness, bearing scars on its paving stones and gashes on its walls, frightful.”

In the 1850s, Georges-Eugène Haussmann, the famous city planner under Napoleon III, gave the sewers a full renovation. He gutted the city streets and laid four hundred miles of new pipes. Engineers installed each segment of pipe at a slope of three centimeters for every meter — gradual enough to allow for easy walking, steep enough to ensure a steady flow. In a series of tests, they discerned that an animal carcass ran the length of the city in eighteen days, while confetti made the same trip in six hours. But no amount of renovation could soften the public’s aversion. Outside of sewer workers — the égoutiers — who spent their days scrubbing muck out of the pipes, no one willingly entered the sewers.

*

We’d been in the sewers maybe ninety seconds when the call came from Steve at the front of the line — “Rat!”

Gray and bandicoot-sized, it skittered up the stream of wastewater at our feet. We all leaped up and straddled the sides of the pipe as it ran under us, sweeping its tail, kicking up a V-shaped wake.

Our route north would take us up the collector beneath boulevard de Sébastopol, a large, circular, brick-lined channel, flanked by two thick water pipes — one carrying potable water, the other nonpotable. It was into the collector that all of the smaller tributary pipes flowed. Down the center ran a recessed canal called a cunette — four feet across, misted over with vapor — which carried a flow of every conceivable form of matter rejected by the surface. A one-minute game of “I Spy”: a syringe, a dead bird, a soggy metro ticket, a chopped credit card, a wine label, a condom, a coffee filter, many globs of toilet paper, as well as floating pieces of shit. “Sewer fresh,” said Moe, fresh being urban explorer lingo for “human excrement.”

Just as we were gearing up — Liz squirting hand sanitizer on everyone’s hands, Moe powering up his gas detector — Steve called for our attention.

He’d received a text message from Ian, our weather sentinel.

“RAIN FORECAST, POSSIBLE THUNDERSTORM, LOOKS TO BE WET.”

Steve went around the circle, looking at us one by one, but there was no hesitation. We were thirty-one hours deep: we’d come too far to give up.

“We just have to be vigilant,” said Steve. As long as we kept a close eye on the level of water in the cunette, he said, and the water coming out of the tributary pipes, we’d be fine.

Steve was more familiar with the trappings of sewers than just about anyone on the planet, which was both comforting and unnerving, because he could narrate in sumptuous detail exactly what would happen to us in the event of a rainstorm. On the slimy wall of the collector, he traced a little graph with his finger, showing the exponential rate at which water would rise. “I’ve walked collectors in New York City, London, Moscow,” he said. “But the flow in Paris is the burliest I’ve seen. Up to your shins, your knees, your waist, before you even realize it. The second we see water start to rise, we just bolt like crazy to the nearest ladder.”

As we headed up the collector, no one spoke. I was practically tiptoeing: the catwalk was slippery and my shoes had almost zero traction. The air was jungle-thick, with burbles and gargles and belches rising up all around us, the sounds of Paris metabolizing. The stink was subtler than you’d imagine — the smell of a refrigerator that needed cleaning — but nonetheless a smell that would cling to you. At the dark junctions were Piranesian traps of slimy ducts and valves. Passing under one rig, maybe fifteen feet up, I saw small ribbons of tattered toilet paper — evidence that a flood had recently gushed down this very pipe.

At one point, a jet of water burst out of a tributary, sending a shock of echoes down the collector. We all froze, wide-eyed, preparing to dash to the nearest ladder.

“Nothing to worry about,” said Steve. An early riser in one of the apartments above had flushed. “Everything down here is magnified,” he reminded us. “Even a little spray is going to sound like Niagara.”

*

Nadar began exploring sewers shortly after his visits to the catacombs. Over the course of several weeks, he navigated the city’s digestive system, his assistants lugging his equipment up and down the catwalks. Compared to the catacombs, the sewers presented far more logistical challenges. Here he was exposed to every vicissitude on the surface, every rain shower and dispersal of toilet water, making it more difficult to find the necessary eighteen minutes of uninterrupted stillness. Each time Nadar opened the shutter, he and his crew would pray that nothing would disrupt the shot. “At the moment when all precautions had been taken,” Nadar later wrote, “all impediments removed or dealt with, the decisive moves being about to take place — all of a sudden, in the last seconds of the exposure, a mist arising from the waters would fog the plate — and what oaths were issued against the belle dame or bon monsieur above us, who without suspecting our presence, picked just that moment to renew their bath water.”

Nadar’s photographs of the sewers revealed the shadowy pipes with a romantic shimmer. Some featured the bearded mannequin, now sporting the coveralls of the égoutier, propped up in the poses of labor. Other images were abstract, focusing on geometric lines: a pipe splitting into two channels, or a current of sewage flowing in a ghostly blur. Due to the vapor in the pipes, each photograph bore a faint haze, as though cast behind a veil.

As we emerged, pedestrians on the sidewalk halted and jumped back, a waiter at the restaurant dropped a fork and knife.

Journalists and reviewers again fawned over the images. A newspaper portrayed Nadar as a pioneer, fighting off peril and treachery in the city’s long-maligned subterranean wilderness, making photographs despite being “half-asphyxiated by noxious gases from the electric battery in those suffocating vaults.” The philosopher Walter Benjamin described them as “the first time that the lens is given the task of making discoveries.”

All over Paris, people began popping sewer manholes. Late at night, they’d climb down, light a candle, and go for a stroll. An 1865 account of a midnight gambit in La Vie Parisienne imagined the sewers as a new promenade. “There are charming encounters to be had there. I encountered the pretty comtesse de T–– — , more or less alone, I also saw the marquise D–– — , and I rubbed elbows with Mlle N–– — of the Variétés theater.” The day would come, the writer predicted, when the allure of sewers eclipsed that of the city’s verdant parklands. “When it becomes possible to tour the sewers on horseback,” he wrote, “the Bois de Boulogne will undoubtedly be deserted.”

During the Exposition Universelle of 1867, the city opened the sewers to official tours, and visitors flocked from all over Europe. Dignitaries and royals, diplomats and ambassadors descended by an iron spiral staircase near the Place de la Concorde and boarded a wagon otherwise used by sewermen to clean the pipes. “A chariot with cushioned seats, its corners illuminated with oil lamps,” recalled one visitor. Ladies in bonnets and high heels, holding lacy umbrellas, glided through the city’s ejecta. The sewer workers played the part of gondoliers, pulling the boat down the canal. “Everyone knows,” recorded a contemporary travel guide, “that no foreigner of distinction wants to leave the city without making this trip.”

Meanwhile, Nadar embraced his role as the Hermes of Paris, the psychopomp, interlocutor between above and below. In the years after his photographs were released, he was known to give private tours of the sewers and quarries, leading tittering groups through the dark. In an essay accompanying his images, the photographer beckoned the masses to follow him into the depths. “Madame,” he wrote, addressing one of his followers, “allow me to be your guide. Please take my arm and suivons le monde.”

*

Before the final stretch, we camped on the banks of an underground section of the Canal Saint-Martin: a broad, arched tunnel where green water flowed placidly, as filmy morning light filtered through the far end. It was about 8 a.m. — on the surface, brasseries would soon be opening, waiters arranging silverware on tables. We strung our hammocks to the railing along the canal, like alpinists in cliffside bivouacs. Steve volunteered to stay up and keep watch.

As I lay in my hammock, thinking of Nadar’s photographs, I recalled a moment from the myth of Phaëthon, the young man who convinces his father, Helios, to let him drive the flaming sun chariot across the sky. Soon after lifting off, the boy loses control of the horses’ reins: the vessel swerves toward the earth, the heat dries up rivers, creates deserts, sets fire to mountaintops, until, finally, Phaëthon flies so low that the chariot burns a hole clear through the earth’s surface, allowing light to pour into the underworld. People scramble to the edge of the hole, where they find that they can see, for the first time, straight down into the sweep of Hades, from the fire-ringed lakes and the gloomy Asphodel Meadows to the endless black of Tartarus. They even see King Hades and Queen Persephone, seated on their thrones, blinking up at them. The people are terror-struck by this infernal landscape, which they have so long dreaded; and yet, they don’t retreat from the edge of the hole. They continue to peer into the gloom, unable to look away.

We’d been asleep maybe two and half hours when Steve spotted a tour boat gliding down the canal. Before the boat captain spotted us and called the police, Steve shook everyone awake and we slipped back into the dark.

Our final stretch was the collector under avenue Jean Jaurès — a long, squared-off corridor, broad and plain, with a current of sewage the width of a one-lane road roaring down the middle. According to Steve, we were walking the main line: almost all of the wastewater in Paris was flowing at our feet.

Now at the thirty-eight-hour mark, we could all feel our destination near. We might have felt triumph, or relief, or a sense of accomplishment, but we were dragging our feet, eyes bloodshot, all of us haggard and a touch dazed, I suspect, from whatever subterranean miasma we’d inhaled in the preceding hours and miles.

“I say we push to the north of France,” Steve said.

Down the slick catwalk, I could feel my eyelids starting to droop: I kept as close to the wall as possible and focused on putting one foot in front of the other. Every few hundred feet, we passed a small tributary pipe marked with the sign of the street above it. Moe, walking in front with the map, called out each street name, and counted off the distance until our destination.

“Five hundred meters!”

With every step, the gush of the canal grew stronger, the sewage splashed over the edges of the catwalk, before it was lapping at the tops of our shoes — the underground was pushing us out.

*

We came to the surface under a bright midday sun, just beyond the city limits: six of us ascending a ladder to emerge through a manhole at the foot of a Turkish restaurant. Our faces were smudged, our hair matted with muck and slime, our clothes soggy and fetid. As we emerged, pedestrians on the sidewalk halted and jumped back, a waiter at the restaurant dropped a fork and knife. An old woman in a pink sweater leaned on her walker and stared down at us, eyes wide, her mouth in a perfect O. And just for a moment — before Steve pulled the manhole cover back into place, before we all stumbled into a nearby park and slashed open a celebratory bottle of champagne — everyone on the street was leaning forward to peer down into the open manhole.

* * *

Will Hunt’s writing, photography, and audio storytelling have appeared in The Economist, The Paris Review Daily, Discover, The Atavist Magazine, and Outside, among other places. He is currently a visiting scholar at the NYU Institute for Public Knowledge.

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